It has been known in the prior art to prepare phosphates of metals of Group II of the Periodic Table, usable as catalysts for selective cleavage of C--O--bonds in organic compounds, and, specifically, for converting 4,4-dimethyl-1,3-dioxane into isoprene, as well as for dehydration of alcohols, by precipitating normal phosphates of Group II metals from aqueous solutions of their salts taken in conjunction with water-soluble salts of phosphoric acid, followed by separating the precipitate, washing the paste obtained, and shaping it into catalyst granules (cf. U.S. Pat. No. 3,872,216).
Catalysts prepared by the above technique, however, are characterized by low selectivity (78-82 mole %), low activity resulting in low DMD space velocity (0.7 h.sup.-1), and high operating temperatures (ca. 375.degree. C.).
Selectivity is defined as the ratio of the amount in moles of isoprene formed to the amount in moles of DMD converted.
Selectivity is quantitatively dependent upon catalyst composition and structure, as well as upon the process conditions under which the catalyst operates.
Increased selectivity will lead to reduced stockfeed (DMD) consumption rates per unit of finished product, thus per ton of isoprene. The relatively low selectivity of the catalyst obtainable by the aforesaid prior technique would result in high feedstock consumption rates in isoprene production, varying between ca. 2.10 and 2.25 kg DMD per kg of isoprene.
The activity of calcium phosphate catalysts is dependent upon their acidity which is determined by the number and efficiency of the active centres and can be characterized by the DMD conversion degree.
DMD conversion is defined as the ratio of the amount of DMD converted to that of DMD used, expressed in percent.
There is also known in the art to produce calcium phosphate catalysts by reacting a calcium salt with a phosphoric acid salt in aqueous ammonia; followed by washing and drying the resulting precipitate and heat treatment with the use of super-heated steam or a mixture of steam and air at high temperatures (cf. U.S. Pat. No. 3,846,338).
The catalysts obtainable by this prior art technique are relatively low in activity.
Furthermore, calcium phosphate catalysts prepared by the aforesaid technique have a low efficiency (0.3 to 0.4 ton/h of isoprene per cubic meter of catalyst).
The efficiency of a catalyst depends on its activity and selectivity, as well as on the feedstock space velocity.
In the prior technique, heat treatment is carried out at high temperatures, which involves overheating of the heat carrier to temperatures as high as 650.degree. to 800.degree. C. and high process power inputs, as well as special heat-resistant materials for reactors adapted to produce the catalyst.
One further disadvantage associated with the catalyst obtainable by the aforesaid technique is a relatively short service life (250 hours).
The catalyst life depends on many factors including catalyst composition and structure, catalyst activity, operating temperatures, and coke deposition. Coke deposition is understood to denote coke deposits on the catalyst in the process of DMD decomposition. It is determinable as the ratio of the amount in moles of coke deposited to the amount in moles of DMD converted, expressed in percent.
In spite of the advantages inherent in the prior art technique for preparation of calcium phosphate catalysts, no commercial process based on said technique has been developed so far, since there is no catalyst as yet, with selectivity and stability such as to justify a commercial process.